There has long been a need for a relatively inexpensive, strong, high performance, permanent magnet. Such high performance permanent magnets would be characterized by relatively high magnetic parameters, e.g. coercive force (H.sub.c) or coercivity, remanent magnetization or remanence, and maximum energy product.
Moreover, an ideal high-performance permanent magnet should exhibit a square magnetic hysteresis loop. That is, upon application of an applied magnetic field H greater than the coercive force Hc, all of the microscopic magnetic moments should align parallel to the direction of the applied force to achieve the saturation magnetization Ms. Moreover, this alignment must be retained not only for H=0 (the remanent magnetization Mr), but also for a reverse applied magnetic force of magnitude less than Hc. This would correspond to a maximum magnetic energy product (the maximum negative value of BH) of EQU (Mr.sup.2 /4)=(Ms.sup.2 /4)
Unfortunately, this ideal situation is at best metastable with respect to the formation of magnetic domains in other directions, which act to reduce Mr and BH.sub.max.
Conventional high-performance permanent magnets that approach square-loop behavior have four general requirements:
1. The material must be composed primarily of a ferromagnetic element or compound with a Curie temperature Tc that significantly exceeds the application temperature Ta, and with Ms at Ta large. Practically speaking, this requires either Fe or Co as the major constituent.
2. In order to obtain a high coercive force, the material must consist of an assembly of small particles or crystallites.
3. These particles or crystallites must exhibit microscopic magnetic anisotropy, i.e. they must have a preferred "easy axis" of magnetization. This can follow either from shape anisotropy or magneto-crystalline interaction.
4. These microscopically anisotropic particles must be aligned substantially in parallel within the macroscopic assembly, in order to achieve values of Mr that approach Ms, i.e. square-loop behavior.
The prior art teaches that good permanent magnetic materials, e.g., having maximum magnetic energy products of about 15 megagaussoersteds, consist of a conglomeration of non-interacting substantially crystallographically oriented uniaxial particles. When a sufficiently large magnetic field is applied in a given direction, the individual vector magnetizations of each of these particles point along the applied field, corresponding to the maximum or saturation value of the net magnetization, M.sub.s. As the applied magnetic field is reduced to zero, the vector magnetization of each particle relaxes back to the easy magnetic axis of the particle, so that the net resultant remanent magnetization, M.sub.r, may be less than M.sub.s.
This is more fully elucidated by the following geometrical model, in which the "easy axis" of magnetization lies along a preferred axis, c. For an isolated uniformly magnetized particle, the magnetization vector, M, lies along the c axis for a zero applied field. If a field is applied in an arbitrary direction z, the magnetization is rotated away from the c axis until, at sufficiently large fields, M is parallel to z and M.sub.z is equal to M.sub.s. When the field is removed, the magnetization relaxes back parallel to the c axis, subject to the condition that the projection of magnetization along the c axis is positive.
E. C. Stoner and E. V. Wohlfarth, Phil. Trans. Royal Soc. (London), A. 240, 599 (1948) have calculated the hysteresis loop for such a particle for different orientations of the c axis with respect to z. For the case of a sample comprising a large number of such non-interacting particles oriented along some direction, the magnetic properties for the material or sample are the sum or average of the properties of the individual particles. Such a sample or material is hereinafter referred to as an anisotropic material. Anisotropic materials have at least one magnetic property which is a strong function of the direction of measurement. Such materials are characterized by a single "easy direction" of magnetization, where the value of the property greatly exceeds the value in other directions of magnetization. If the particles are non-interacting, the maximum energy product varies from a maximum value of 0.25 (M.sub.s).sup.2, when z is parallel to the c axis, to 0 when z is perpendicular to the c axis. For a theoretical anisotropic material with M.sub.s equal to 16 and H.sub.c chosen to be greater than M.sub.s, the maximum theoretical value of the energy product of the hysteresis loop is 64 megagaussoersteds.
Stoner and Wohlfarth have carried out the same method of analysis for an ideal array of randomly oriented non-interacting uniformly magnetized particles. Since the array is isotropic there is no dependence of the hysteresis loop on the direction of the applied field. The maximum theoretical value of the energy product of such a loop is dependent on M.sub.s and H.sub.c. If M.sub.s is chosen to equal 16 kilogauss and H.sub.c is chosen to be much greater than M.sub.s, then the maximum energy product is 16 megagaussoersteds.
Hence, the teaching of the prior art for a perfectly oriented non-interacting material (anisotropic) is that the maximum energy product is at least four (4) times that of the same material when randomly oriented (isotropic).
For a general distribution of orientations of non-interacting particles, as a consequence of simple vector geometry, EQU (M.sub.r /M.sub.s)=[cos(theta)],
where theta is the angle between the applied field and the easy axis of a given particle, and the result, indicated by double brackets, represents the size weighted average over all of the particles. As is well understood in the art, M.sub.r /M.sub.s =1 along the direction of orientation of a perfectly oriented, non-interacting, permanent magnet sample (anisotropic), and M.sub.r /M.sub.s =0.5 in all directions for a completely unoriented, non-interacting sample (isotropic). See, e.g., R. A. McCurrie, "Determination of the Easy Axis Alignment in Uniaxial Permanent Magnets for Remanence Measurements", J. Appl. Phys., Vol. 52, (No. 12), pages 7344-7346 (December 1981). Observations in the literature are consistent with this prediction. See, e.g., J. F. Herbst and J. C. Tracy, "On Estimating Remanent Magnetization from X-Ray Pole Figure Date", J. Appl. Phys., Vol. 50 (No. 6), pp. 4283-4284 (June 1979).
A figure of merit, which applicants refer to as the magnetic retention parameter, is EQU Q=Sum.sub.x,y,z (M.sub.r M.sub.s).sup.2,
where M.sub.s and M.sub.r are measured with the applied magnetic field along three orthogonal directions. Theoretically, for magnetic materials of the prior art, Q approaches 1 for perfectly oriented, non-interacting, particles or crystallites (anisotropic) and 0.75 for completely unoriented, non-interacting, crystallites (isotropic). The behavior for reported values of permanent magnetic materials of the prior art tend to produce values of Q which are substantially below the theoretical values. See, e.g., McCurrie; Herbst and Tracy; and Stoner and Wohlfarth; above.
Prior art systems which are non-interacting and conform to the assumptions of and models in Stoner and Wohlfarth are described in the Background sections of commonly assigned copending U.S. application Ser. No. 816,778, filed Jan. 10, 1986, of R. Bergeron, R. McCallum, K. Canavan, and J. Keem for Enhanced Remanence Permanent Magnetic Alloy Bodies and Methods of Preparing Same, and U.S. application Ser. No. 893,516, filed Aug. 5, 1986 of R. Bergeron, R. McCallum, K. Canavan, J. Keem, A. Kadin, and G. Clemente, for Enhanced Remanence Permanent Magnetic Alloy and Bodies Thereof. The prior art materials described and discussed in the Background sections of our earlier applications do not exhibit any deviations from the assumptions and models of Stoner and Wohlfarth.
Deviations from (M.sub.r M.sub.s ) =[Cos(theta)]corresponding to larger values of Mr might be expected to occur if the particles were permitted to interact with one another. Suggestions of this sort have appeared in the magnetic recording literature, where the proposed interaction was due to long range magnetic dipole fields. See, for example, H. N. Bertram and A. K. Bhatia, The Effect of Interaction on the Saturation Remanence of Particulate Assemblies, IEEE Trans. on Magnetics, MAG-9, pp 127-133 (1983), and R. F. Soohoo, Influence of Particle Interaction on Coercivity and Squareness of Thin Film Recording Media, J. Appl. Phys., Vol 52(3), pp 2459-2461 (1981). However, this assumption of interactions has been questioned. See, for example, P. M. Davis, Effects of Interaction Fields on the Hysteretic Properties of Assemblies of Randomly Oriented Magnetic or Electric Moments, J. Appl. Phys., Vol 51 (2), pp 594-600 (1980).
Suggestions of short range interactions based on exchange have also been made with respect to amorphous iron-rare earth alloys at cryogenic temperatures by E. Callen, Y. L. Liu, and J. R. Cullen, Initial Magnetization, Remanence, and Coercivity of the Random Anisotropy Amorphous Ferromagnet Phys. Rev. B, Vol. 16, pp 263-270 (1977).
The literature does not contain any verified indications of enhanced values of Mr relative to those predicted by Stoner and Wohlfarth, above, in isotropic permanent magnetic materials.
However, contrary to the limited but negative teachings of the prior art interaction between crystallites has been used to achieve enhanced magnetic properties in bulk solid materials. Magnetic materials which utilize interaction are described in commonly assigned copending U.S. application Ser. No. 816,778, filed Jan. 10, 1986, of R. Bergeron, R. McCallum, K. Canavan, and J. Keem for Enhanced Remanence Permanent Magnetic Alloy Bodies and Methods of Preparing Same, and U.S. application Ser. No. 893,516, filed Aug. 5, 1986 of R. Bergeron, R. McCallum, K. Canavan, J. Keem, A. Kadin, and G. Clemente, for Enhanced Remanence Permanent Magnetic Alloy and Bodies Thereof, both of which are incorporated herein by reference.
Described therein is a class of permanent magnetic alloys which exhibit superior magnetic properties as measured in all spatial directions, that is, isotropically. The magnetic parameters are of a magnitude which the prior art teaches to be only attainable in one spatial direction, that is, anisotropically, and to be only attainable with aligned materials.
The magnetic materials described in the incorporated patent applications have a ratio of net remanent magnetization (M.sub.r) to net saturation magnetization (M.sub.s), exceeding 0.5 and approaching 1.0, in all directions, without any significant preferred crystallite orientation. This is a clea violation of the consequences of the Stoner and Wohlfarth's model and the assumptions of the prior art that the grains must be microscopically anisotropic grains that are aligned substantially in parallel within the macroscopic body in order to achieve values of Mr approaching Ms, i.e., square hysteresis loop behavior.
These permanent magnetic materials have isotropic magnetic retention parameters, Q, as described above, greater than 0.75 and preferably greater than 1. The theoretical limit of the magnetic retention parameter, Q, for the herein contemplated materials is believed to approach 3, rather than the theoretical values of 1.0 and 0.75 respectively, for aligned (anisotropic) and unaligned (isotropic), non-interacting materials of the prior art.
Ribbon samples of the as quenched materials described above, without further processing, exhibit remanent magnetization, M.sub.r, greater than 8 kilogauss, coercive force, H.sub.c, greater than 8 kilooersteds, and preferably greater than 11 kilooersteds, and maximum energy product (BH)max greater than 15 megagaussoersteds with similar values measured in all directions, i.e., in the plane of the ribbon and perpendicular to the plane of the ribbon. In the latter case the value was obtained after a standard correction (a geometric demagnetization factor as described, for example, in R. M. Bozorth, Ferromagnetism, D. VanNostrand Co., New York, (1951), at pages 845-847) for the shape anisotropy of the ribbon.
The saturation magnetization M.sub.s of the ribbon, i.e., the magnetization in the limit for large applied fields, e.g., an applied magnetic field above about 50 kilogauss, is 15 to 16 kilogauss, also in all directions. In order to directly measure saturation magnetization, M.sub.s, the applied field should be at least three times the coercive force, H.sub.c. Alternatively, the value of M.sub.s can be estimated based on the values thereof for compositionally similar materials. The values correspond to a value of M.sub.r /M.sub.s greater than 0.5, and a magnetic retention parameter, Q, greater than 0.75, in contradistinction to the clear teachings of the prior art for a macroscopically isotropic, non-interacting material.
Typical magnetic parameters for the magnetic alloys described in the above incorporated patent application are as shown in Table I of U.S. application Ser. No. 893,516, filed Aug. 5, 1986, Table V of U.S. patent application Ser. No. 816,778. (An M.sub.s of 16 kilogauss was used.)
As can be seen from Table I of U.S. application Ser. No. 893,516, filed Aug. 5, 1986, the samples of the materials described therein exhibit superior relevant magnetic parameters throughout the volume of the bulk solid, evidencing interaction between grains. The properties are especially superior when compared with the properties of the isotropic materials of the prior art listed in Table III of U.S. application Ser. No. 816,778. When compared with the anisotropic prior art materials listed in Table IV of U.S. application Ser. No. 816,778, the samples of the inventions described in the aforementioned U.S. patent application Ser. Nos. 816,778 and 893,516 (filed Aug. 5, 1986) exhibit comparable but isotropic magnetic properties, and were prepared without the costly, complicated alignment steps necessary in the prior art.
The magnetic alloy materials of U.S. application Ser. Nos. 816,778 and 893,516 (filed Aug. 5, 1986) have been prepared by the melt spinning process, and more particularly by the free jet casting process.
In the free jet casting process a jet of molten metal is expelled under a head of inert gas from a crucible onto a rapidly rotating chill wheel. This jet of molten metal forms a puddle of molten metal on a rapidly rotating chill wheel. The top of the puddle appears to stand stationary beneath the orifice of the crucible, while the bottom of the puddle appears to be continuously drawn away from the crucible orifice. We have observed an instability associated with the interaction between the chill wheel and the puddle. This instability is associated with a high degree of variance of magnetic properties of the cast products and a concommitant low yield of enhanced remanence magnetic alloy material.